1. Field of the Invention
The present invention relates to the field of medical ultrasound imaging. More specifically, it involves an imaging system which incorporates techniques first developed in the geophysical sciences for analyzing seismic traces to create high resolution medical images.
2. Description of the Related Art
Well known ultrasound imaging systems commonly provide medical images for a variety of uses. In general, ultrasound imaging utilizes a sound source to ensonify a target area and a detector to receive the echo. Ultrasound can often penetrate objects which are impervious to electromagnetic radiation. However, acoustical imaging systems have traditionally produced lower resolution images than electromagnetic radiation imaging systems.
The introduction of synthetic aperture focusing into medical imaging systems has helped to improve the quality of ultrasound images over previous systems which used direct focusing. The synthetic aperture focusing technique (SAFT) was originally used in radar systems to image large areas on the ground from an aircraft. In SAFT, the transmitted beam is a broad band signal which is sent out in a wide cone of transmission. The broadband nature of the transmitted signal allows direct measurement of the time of return or the phase information of the signal, thereby allowing the determination of the range of any reflectors (i.e., changes in acoustical impedance) which cause returning echoes. Moreover, the wide cone transmission beam in SAFT allows recording on one receiver channel of echoes returning from all directions. This provides information about an entire area, eliminating the need to scan the area point by point. However, since the direction from which the echoes have come cannot be determined from one receiver channel, SAFT requires the use of many receiver channels. By comparing information across all of the channels, the direction of the returning signal can be determined. This process enables focusing on a point by analyzing the receiver traces, and is thus referred to as synthetic focusing.
Another advantage of SAFT is that since it does not require beam steering, there is no need for expensive hardware to drive the array. Additionally, because the wavelength of the radiation used does not limit resolution, the resolution of the images produced can be increased by many orders of magnitude over that obtained with direct focusing. Examples of ultrasonic imaging systems that use SAFT are disclosed in U.S. Pat. Nos. 3,548,642 and 3,895,381.
U.S. Pat. No. 4,325,257 to Kino, et al. discloses a more recent acoustic imaging system that exemplifies typical SAFT processing. The Kino patent describes using an array of transducer elements in which each element is multiplexed in sequence to emit an ultrasonic pulse into the sample. Each transmitting element then acts as a receiver which measures and records the returning echoes. Once all of the transducer elements have obtained a time history trace of the echoes (i.e., a record of the return beam for a selected period of time), the traces are transformed into a three-dimensional image of the target using a conventional reconstruction algorithm. Each point of the three-dimensional image represents a point in the sample and contains a value which represents the strength of the reflected signal at that represented point location in the sample. Strong reflectors, such as bone, have high values at the surface. Values are close to zero at locations where there are no reflecting surfaces or objects. Once a three-dimensional image is obtained, it can be collapsed to generate any two-dimensional view of the sample using conventional tomography techniques. Typical systems display the collapsed two-dimensional view on a CRT monitor.
The reconstruction algorithm disclosed in the Kino patent is based on the travel time of the echo signals. In other words, for a reflecting object at a given location in a sample, the echo returning from that reflector appears at a different time in the time history trace of each receiver channel. The algorithm involves calculating, for a specified reflector location, the return time to each receiver from that specified reflector location, and then summing across the channels all of the echoes which came from that specified location in the sample. The summing reinforces any coherent information from a potential reflector at the specified location and cancels the noise from various other random locations in the sample, leaving only the signal information which originated from the specified location. If no reflector (i.e., no acoustical impedance change) is present at the specified location, no coherent information will exist and the signals tend to cancel each other when summed. Each point location in the three-dimensional map (also known as a focus map) of the sample is calculated using this procedure. This procedure is commonly termed "migration" in geophysics, and there is a great amount of literature published about it dating back to 1954, when J. L. Hagedoorn's thesis paper entitled "A Process of Seismic Reflection Interpretation," provided the graphical foundation on which migration procedures are based. This paper can be found in Geophysical Prospecting, Volume II, No. 2, June 1954.
U.S. Pat. Nos. 5,005,418 and 4,817,434 to Anderson both disclose medical ultrasound imaging systems that incorporate SAFT using a single transmitted pulse. The systems described use a transducer array having a center transmitter element and a set of surrounding receiver elements. Instead of multiplexing the receiver channels to record a set of return signals in sequence, the transmitter sends out a single pulse, and the receivers record the returning echoes simultaneously. From the recorded time-history traces, a focus map of the sample is obtained using a reconstruction algorithm. U.S. Pat. No. 4,817,434 discloses a summing algorithm similar in principle to the one described by Kino, except that all of the time history traces originate from the same transmitter. This is similar to algorithms that are used in seismic exploration which are known as migration algorithms. U.S. Pat. No. 5,005,418 discloses a reconstruction algorithm known as ellipsoidal backprojection, which differs from the procedure described in U.S. Pat. No. 4,817,434. However, the backprojection algorithm also relies on the time-of-travel principle.
Because the reconstruction methods described above rely on time-of-travel calculations, calculating the correct travel time between the array elements and the reflector locations in the sample for the purpose of reconstructing the three-dimensional map of the sample requires knowledge of the velocity of sound typically through the sample. A problem that arises with both ellipsoidal backprojection and migration, which both rely on time-of-travel calculations, is that the velocity of sound varies at different locations throughout the sample. Knowledge of these velocity variations provides information needed to correctly align the receiver traces for the summing process.
However, because the velocity variations are not known in advance, the reconstruction algorithms disclosed in the conventional systems rely on an assumption that the velocity of sound does not vary throughout the sample. This assumption seriously limits obtaining accurate information about reflector locations. The sound may also refract as it travels through the sample, thereby increasing the time-of-travel as well as changing the receiver location (from that expected) of the first return signal from a reflector in the sample.
In general, current techniques in medical imaging do not adequately account for either of these effects. These unaccounted for realities severely degrade the accuracy and quality of reconstructed images because the coherent reflector information from any selected channel will be skewed in time from coherent information from other channels. The skew in time is caused by the velocity variations within the sample. This results in a significant loss of information when the signals are summed together because coherent information which is skewed in time will be discarded as noise in the summing process, and noise signals may be summed as coherent information. Thus, a need exists for a more satisfactory reconstruction procedure which accounts for the changes in sound velocity throughout the sample.
The geophysical sciences utilize reconstruction methods in seismic imaging to accurately obtain velocity information. Determining the location of reflecting surfaces beneath the ground and identifying the various geological materials of which the strata is composed are both important in geology. A technique commonly called common depth point (CDP) stacking in seismic imaging determines the velocity of sound for different travel paths and different locations throughout the sample. The velocities provide accurate information for calculating the correct time-of-travel in the migration procedure. The velocities can also be compared with a database of velocities to identify the nature of materials at various locations. W. Harry Mayne introduced CDP stacking in 1962 in the field of seismography. (see Geophysics, Vol. 27, no. 6, p. 927). More recent uses of this method are disclosed in U.S. Pat. No. 4,992,996 to Wang, et al.